In the field of electrophotographic color printing, one conventional approach to developing a color image on an organic photoconductor and then transferring the developed color image to an adjacent print medium is to use a so-called intermediate transfer member (ITM) which is located between a surface of the organic photoconductive drum and the surface of the print medium. Using this approach, liquid toners of cyan, yellow, magenta, or black are first transferred electrostatically in series from a conventional source of liquid toners to the surface of the organic photoconductor and then serially developed thereon such as by writing the desired color image with a controlled laser beam or other suitable light source. Color liquid toners are generally well known in the art of electrophotographic printing and are described in some detail, for example, in U.S. Pat. Nos. 4,925,766 and 4,946,753 issued to Elmasry et al and entitled "Liquid Electrophotographic Toners", both incorporated herein by reference.
When each color of cyan, yellow, magenta, or black has been individually developed on the organic photoconductive drum, the intermediate transfer member is then brought into intimate contact with the surface of the drum and is rotated against the drum to thereby serially transfer each color image from the surface of the photoconductive drum to the intermediate transfer member. Since the color toners used in this process could not be directly transferred to the media, each color toner had to be first developed on the photoconductive drum and then stored on the intermediate transfer member where subsequently developed color images would be superimposed one upon the other. When all of the cyan, magenta, yellow, and black toner images were formed on the intermediate transfer member, the print media was brought into intimate contact with the intermediate transfer member for transferring the composite developed color image thereto using a combination of heat and mechanical pressure supplied by conventional transfer roller techniques. One example of using an intermediate transfer member (ITM) for transferring the color image from the photoconductive drum to the print media is disclosed in U.S. Pat. No. 4,286,039 issued to Landa et al. and incorporated herein by reference. In addition, an ITM color printer of the type generally described above has also been described by the 3-M Company of Minneapolis, Minn. and has been labeled as their "Digital Matchprint or Digital Writer", but the constructional details of this color printer are not currently known.
The use of the above described intermediate transfer member has been required in the above prior art electrophotographic color printing apparatus because of the fact that intimate contact between the toner and the receiving surface was essential for high quality transfer of the developed image from the surface of the photoconductive drum. The requirement for the use of this intermediate transfer member had the effect of adding production cost and maintenance expense to these color printers and required critical transfer alignments and relative movements between the intermediate transfer member and the photoconductive drum in order to provide an acceptable print quality on the printed media.
Subsequently, when transparent color toners were being developed, proposals were made to eliminate the above intermediate transfer member and develop the color images directly on top of one another on the surface of the photoconductive drum. However, earlier attempts to transfer developed transparent color images directly from the surface of the photoconductive drum to the print media failed because there were not sufficient transfer forces and/or good intimate contact made between the developed color images on the drum and the relatively rough surface of the print media as a result of the conventional heat and pressure transfer roller techniques utilized. It is the solution to this latter problem to which the present invention is directed.